My two cents (supplied free of charge) - clubbing the guy over the head about what a cretin he is for his opinion is counterproductive. Better to politely illustrate that his opinion is founded on some seriously one-sided, narrow-minded extremist idealism that fails to properly and objectively consider all of the facts involved. I find that many people of even the most extreme views (on both sides) are more susceptible to having their beliefs altered through thoughtfully expressed opinions to the contrary than their outward online persona may reflect.

“The greatness of a nation and its moral progress can be judged by the way its animals are treated.” - Mahatma Gandhi